It has been frequently proposed to increase the fuel efficiency of a multi-cylinder internal combustion engine at light loads by disabling certain of the cylinders so that they do not burn fuel. For example, an eight cylinder automotive engine can be operated very satisfactorily on four cylinders when the vehicle is cruising at highway speeds, idling, or decreasing in speed. The full load output of the engine may be as much as 200 horsepower and yet the demand at cruising speeds may be only 30 or 40 horsepower. If four of the cylinders draw no fuel, the light load can be economically absorbed at a fuel saving approaching 40 percent. The driver notices no difference in smoothness of operation and if the four disabled cylinders can be reactivated at will or automatically when the load demand increases there is no overall loss of performance.
The various devices proposed to perform the desired function have either required extensive engine modifications (e.g. shiftable cam shafts) or have not been sufficiently rapid in response to be practicable.